Change in the lithium isotope composition in a lithium cobalt oxide (LiCoO2) cathode for lithium ion secondary batteries accompanying the electrochemical lithium release from the cathode into an organic electrolyte solution was observed. The 7Li/6Li isotopic ratios of the electrodes after the release of 37.2 to 55.4 % lithium were 1.018 to 1.033 times smaller than that before the release. This means that the heavier isotope, 7Li, is preferentially transferred to the electrolyte solution.